Re: Need help with fixing the Xen waitqueue feature

Re: Need help with fixing the Xen waitqueue feature

[Andres Lagar-Cavilla]

> Date: Tue, 08 Nov 2011 22:05:41 +0000
> From: Keir Fraser <keir.xen@gmail.com>
> Subject: Re: [Xen-devel] Need help with fixing the Xen waitqueue
> 	feature
> To: Olaf Hering <olaf@aepfle.de>,	<xen-devel@lists.xensource.com>
> Message-ID: <CADF5835.245E1%keir.xen@gmail.com>
> Content-Type: text/plain;	charset="US-ASCII"
>
> On 08/11/2011 21:20, "Olaf Hering" <olaf@aepfle.de> wrote:
>
>> Another thing is that sometimes the host suddenly reboots without any
>> message. I think the reason for this is that a vcpu whose stack was put
>> aside and that was later resumed may find itself on another physical
>> cpu. And if that happens, wouldnt that invalidate some of the local
>> variables back in the callchain? If some of them point to the old
>> physical cpu, how could this be fixed? Perhaps a few "volatiles" are
>> needed in some places.
>
>>From how many call sites can we end up on a wait queue? I know we were
>> going
> to end up with a small and explicit number (e.g., in __hvm_copy()) but
> does
> this patch make it a more generally-used mechanism? There will unavoidably
> be many constraints on callers who want to be able to yield the cpu. We
> can
> add Linux-style get_cpu/put_cpu abstractions to catch some of them.
> Actually
> I don't think it's *that* common that hypercall contexts cache things like
> per-cpu pointers. But every caller will need auditing, I expect.

Tbh, for paging to be effective, we need to be prepared to yield on every
p2m lookup.

Let's compare paging to PoD. They're essentially the same thing: pages
disappear, and get allocated on the fly when you need them. PoD is a
highly optimized in-hypervisor optimization that does not need a
user-space helper -- but the pager could do PoD easily and remove all that
p2m-pod.c code from the hypervisor.

PoD only introduces extraneous side-effects when there is a complete
absence of memory to allocate pages. The same cannot be said of paging, to
put it mildly. It returns EINVAL all over the place. Right now, qemu can
be crashed in a blink by paging out the right gfn.

To get paging to where PoD is, all these situations need to be handled in
a manner other than returning EINVAL. That means putting the vcpu on a
waitqueue on every location p2m_pod_demand_populate is called, not just
__hvm_copy.

I don't know that that's gonna be altogether doable. Many of these gfn
lookups happen in atomic contexts, not to mention cpu-specific pointers.
But at least we should aim for that.

Andres
>
> A sudden reboot is very extreme. No message even on a serial line? That
> most
> commonly indicates bad page tables. Most other bugs you'd at least get a
> double fault message.
>
>  -- Keir
>

Re: Need help with fixing the Xen waitqueue feature

[Olaf Hering]

On Tue, Nov 08, Andres Lagar-Cavilla wrote:

> Tbh, for paging to be effective, we need to be prepared to yield on every
> p2m lookup.

Yes, if a gfn is missing the vcpu should go to sleep rather than
returning -ENOENT to the caller. Only the query part of gfn_to_mfn
should return the p2m paging types.

> Let's compare paging to PoD. They're essentially the same thing: pages
> disappear, and get allocated on the fly when you need them. PoD is a
> highly optimized in-hypervisor optimization that does not need a
> user-space helper -- but the pager could do PoD easily and remove all that
> p2m-pod.c code from the hypervisor.

Perhaps PoD and paging could be merged, I havent had time to study the
PoD code.

> PoD only introduces extraneous side-effects when there is a complete
> absence of memory to allocate pages. The same cannot be said of paging, to
> put it mildly. It returns EINVAL all over the place. Right now, qemu can
> be crashed in a blink by paging out the right gfn.

I have seen qemu crashes when using emulated storage, but havent
debugged them yet. I suspect they were caused by a race between nominate
and evict.

Olaf

Re: Need help with fixing the Xen waitqueue feature

[Andres Lagar-Cavilla]

Hi there,
> On Tue, Nov 08, Andres Lagar-Cavilla wrote:
>
>> Tbh, for paging to be effective, we need to be prepared to yield on
>> every
>> p2m lookup.
>
> Yes, if a gfn is missing the vcpu should go to sleep rather than
> returning -ENOENT to the caller. Only the query part of gfn_to_mfn
> should return the p2m paging types.
>
>> Let's compare paging to PoD. They're essentially the same thing: pages
>> disappear, and get allocated on the fly when you need them. PoD is a
>> highly optimized in-hypervisor optimization that does not need a
>> user-space helper -- but the pager could do PoD easily and remove all
>> that
>> p2m-pod.c code from the hypervisor.
>
> Perhaps PoD and paging could be merged, I havent had time to study the
> PoD code.

Well, PoD can be implemented with a pager that simply shortcuts the step
that actually populates the page with contents. A zeroed heap page is good
enough. It's fairly simple for a pager to know for which pages it should
return zero.

PoD also does emergency sweeps under memory pressure to identify zeroes,
that can be easily implemented by a user-space utility.

The hypervisor code keeps a list of 2M superpages -- that feature would be
lost.

But I doubt this would fly anyways: PoD works for non-ept modes, which I
guess don't want to lose that functionality.

>
>> PoD only introduces extraneous side-effects when there is a complete
>> absence of memory to allocate pages. The same cannot be said of paging,
>> to
>> put it mildly. It returns EINVAL all over the place. Right now, qemu can
>> be crashed in a blink by paging out the right gfn.
>
> I have seen qemu crashes when using emulated storage, but havent
> debugged them yet. I suspect they were caused by a race between nominate
> and evict.
>
> Olaf
>

Re: Need help with fixing the Xen waitqueue feature

[Andres Lagar-Cavilla]

Also,
> On Tue, Nov 08, Andres Lagar-Cavilla wrote:
>
>> Tbh, for paging to be effective, we need to be prepared to yield on
>> every
>> p2m lookup.
>
> Yes, if a gfn is missing the vcpu should go to sleep rather than
> returning -ENOENT to the caller. Only the query part of gfn_to_mfn
> should return the p2m paging types.
>
>> Let's compare paging to PoD. They're essentially the same thing: pages
>> disappear, and get allocated on the fly when you need them. PoD is a
>> highly optimized in-hypervisor optimization that does not need a
>> user-space helper -- but the pager could do PoD easily and remove all
>> that
>> p2m-pod.c code from the hypervisor.
>
> Perhaps PoD and paging could be merged, I havent had time to study the
> PoD code.
>
>> PoD only introduces extraneous side-effects when there is a complete
>> absence of memory to allocate pages. The same cannot be said of paging,
>> to
>> put it mildly. It returns EINVAL all over the place. Right now, qemu can
>> be crashed in a blink by paging out the right gfn.
>
> I have seen qemu crashes when using emulated storage, but havent
> debugged them yet. I suspect they were caused by a race between nominate
> and evict.

After a bit of thinking, things are far more complicated. I don't think
this is a "race." If the pager removed a page that later gets scheduled by
the guest OS for IO, qemu will want to foreign-map that. With the
hypervisor returning ENOENT, the foreign map will fail, and there goes
qemu.

Same will happen for pv backend mapping grants, or the checkpoint/migrate
code.

I guess qemu/migrate/libxc could retry until the pager is done and the
mapping succeeds. It will be delicate. It won't work for pv backends. It
will flood the mem_event ring.

Wait-queueing the dom0 vcpu is a no-go -- the machine will deadlock quicly.

My thinking is that the best bet is to wait-queue the dom0 process. The
dom0 kernel code handling the foreign map will need to put the mapping
thread in a wait-queue. It can establish a ring-based notification
mechanism with Xen. When Xen completes the paging in, it can add a
notification to the ring. dom0 can then awake the mapping thread and
retry.

Not simple at all. Ideas out there?

Andres

>
> Olaf
>

Re: Need help with fixing the Xen waitqueue feature

[Olaf Hering]

On Wed, Nov 09, Andres Lagar-Cavilla wrote:

> After a bit of thinking, things are far more complicated. I don't think
> this is a "race." If the pager removed a page that later gets scheduled by
> the guest OS for IO, qemu will want to foreign-map that. With the
> hypervisor returning ENOENT, the foreign map will fail, and there goes
> qemu.

The tools are supposed to catch ENOENT and try again.
linux_privcmd_map_foreign_bulk() does that. linux_gnttab_grant_map()
appears to do that as well. What code path uses qemu that leads to a
crash?

> I guess qemu/migrate/libxc could retry until the pager is done and the
> mapping succeeds. It will be delicate. It won't work for pv backends. It
> will flood the mem_event ring.

There will no flood, only one request is sent per gfn in
p2m_mem_paging_populate().

Olaf

Re: Need help with fixing the Xen waitqueue feature

[Andres Lagar-Cavilla]

Olaf,
> On Wed, Nov 09, Andres Lagar-Cavilla wrote:
>
>> After a bit of thinking, things are far more complicated. I don't think
>> this is a "race." If the pager removed a page that later gets scheduled
>> by
>> the guest OS for IO, qemu will want to foreign-map that. With the
>> hypervisor returning ENOENT, the foreign map will fail, and there goes
>> qemu.
>
> The tools are supposed to catch ENOENT and try again.
> linux_privcmd_map_foreign_bulk() does that. linux_gnttab_grant_map()
> appears to do that as well. What code path uses qemu that leads to a
> crash?

The tools retry as long as IOCTL_PRIVCMD_MMAPBATCH_V2 is supported. Which
it isn't on mainline linux 3.0, 3.1, etc. Which dom0 kernel are you using?

And for backend drivers implemented in the kernel (netback, etc), there is
no retrying.

All those ram_paging types and their interactions give me a headache, but
I'll trust you that only one event is put in the ring.

I'm using 24066:54a5e994a241. I start windows 7, make xenpaging try to
evict 90% of the RAM, qemu lasts for about two seconds. Linux fights
harder, but qemu also dies. No pv drivers. I haven't been able to trace
back the qemu crash (segfault on a NULL ide_if field for a dma callback)
to the exact paging action yet, but no crashes without paging.

Andres

>
>> I guess qemu/migrate/libxc could retry until the pager is done and the
>> mapping succeeds. It will be delicate. It won't work for pv backends. It
>> will flood the mem_event ring.
>
> There will no flood, only one request is sent per gfn in
> p2m_mem_paging_populate().
>
> Olaf
>

Re: Need help with fixing the Xen waitqueue feature

[Jan Beulich]

>>> On 10.11.11 at 05:29, "Andres Lagar-Cavilla" <andres@lagarcavilla.org> wrote:
> The tools retry as long as IOCTL_PRIVCMD_MMAPBATCH_V2 is supported. Which
> it isn't on mainline linux 3.0, 3.1, etc.

Seems like nobody cared to port over the code from the old 2.6.18 tree
(or the forward ports thereof).

> Which dom0 kernel are you using?

Certainly one of our forward port kernels.

> And for backend drivers implemented in the kernel (netback, etc), there is
> no retrying.

As above, seems like nobody cared to forward port those bits either.

Jan

Re: Need help with fixing the Xen waitqueue feature

[Keir Fraser]

On 10/11/2011 04:29, "Andres Lagar-Cavilla" <andres@lagarcavilla.org> wrote:

>> The tools are supposed to catch ENOENT and try again.
>> linux_privcmd_map_foreign_bulk() does that. linux_gnttab_grant_map()
>> appears to do that as well. What code path uses qemu that leads to a
>> crash?
> 
> The tools retry as long as IOCTL_PRIVCMD_MMAPBATCH_V2 is supported. Which
> it isn't on mainline linux 3.0, 3.1, etc. Which dom0 kernel are you using?
> 
> And for backend drivers implemented in the kernel (netback, etc), there is
> no retrying.

Getting this working without a new Linux kernel -- and with
as-yet-to-be-written new stuff in it -- is unlikely to be on the cards is
it?

I think you suggested an in-kernel mechanism to wait for page-in and then
retry mapping. If that could be used by the in-kernel drivers and exposed
via our privcmd interface for qemu and rest of userspace too, that may be
the best single solution. Perhaps it could be largely hidden behind the
existing privcmd-mmap ioctls.

 -- Keir

Re: Need help with fixing the Xen waitqueue feature

[Olaf Hering]

On Wed, Nov 09, Andres Lagar-Cavilla wrote:

> Olaf,
> > On Wed, Nov 09, Andres Lagar-Cavilla wrote:
> >
> >> After a bit of thinking, things are far more complicated. I don't think
> >> this is a "race." If the pager removed a page that later gets scheduled
> >> by
> >> the guest OS for IO, qemu will want to foreign-map that. With the
> >> hypervisor returning ENOENT, the foreign map will fail, and there goes
> >> qemu.
> >
> > The tools are supposed to catch ENOENT and try again.
> > linux_privcmd_map_foreign_bulk() does that. linux_gnttab_grant_map()
> > appears to do that as well. What code path uses qemu that leads to a
> > crash?
> 
> The tools retry as long as IOCTL_PRIVCMD_MMAPBATCH_V2 is supported. Which
> it isn't on mainline linux 3.0, 3.1, etc. Which dom0 kernel are you using?

I'm running SLES11 as dom0. Now thats really odd that there is no ENOENT
handling in mainline, I will go and check the code.

> And for backend drivers implemented in the kernel (netback, etc), there is
> no retrying.

A while ago I fixed the grant status handling, perhaps that change was
never forwarded to pvops, at least I didnt do it at that time.

> I'm using 24066:54a5e994a241. I start windows 7, make xenpaging try to
> evict 90% of the RAM, qemu lasts for about two seconds. Linux fights
> harder, but qemu also dies. No pv drivers. I haven't been able to trace
> back the qemu crash (segfault on a NULL ide_if field for a dma callback)
> to the exact paging action yet, but no crashes without paging.

If the kernel is pvops it may need some audit to check the ENOENT
handling.

Olaf

Re: Need help with fixing the Xen waitqueue feature

[Olaf Hering]

On Thu, Nov 10, Olaf Hering wrote:

> On Wed, Nov 09, Andres Lagar-Cavilla wrote:
> > The tools retry as long as IOCTL_PRIVCMD_MMAPBATCH_V2 is supported. Which
> > it isn't on mainline linux 3.0, 3.1, etc. Which dom0 kernel are you using?
> I'm running SLES11 as dom0. Now thats really odd that there is no ENOENT
> handling in mainline, I will go and check the code.

xen_remap_domain_mfn_range() has to catch -ENOENT returned from
HYPERVISOR_mmu_update() and return it to its callers. Then
drivers/xen/xenfs/privcmd.c:traverse_pages() will do the right thing.
See
http://xenbits.xen.org/hg/linux-2.6.18-xen.hg/rev/0051d294bb60

The granttable part needs more changes, see
http://xenbits.xen.org/hg/linux-2.6.18-xen.hg/rev/7c7efaea8b54


Olaf

Re: Re: Need help with fixing the Xen waitqueue feature

[Andres Lagar-Cavilla]

Thanks Jan, and thanks Olaf for the pointers to specific patches. I'll try
to cherry pick those into my dom0 (debian mainline 3.0). Somebody else
should get those in mainline though. Soonish :)

Andres
>>>> On 10.11.11 at 05:29, "Andres Lagar-Cavilla" <andres@lagarcavilla.org>
>>>> wrote:
>> The tools retry as long as IOCTL_PRIVCMD_MMAPBATCH_V2 is supported.
>> Which
>> it isn't on mainline linux 3.0, 3.1, etc.
>
> Seems like nobody cared to port over the code from the old 2.6.18 tree
> (or the forward ports thereof).
>
>> Which dom0 kernel are you using?
>
> Certainly one of our forward port kernels.
>
>> And for backend drivers implemented in the kernel (netback, etc), there
>> is
>> no retrying.
>
> As above, seems like nobody cared to forward port those bits either.
>
> Jan
>
>

Re: Re: Need help with fixing the Xen waitqueue feature

[Konrad Rzeszutek Wilk]

On Thu, Nov 10, 2011 at 05:57:18AM -0800, Andres Lagar-Cavilla wrote:
> Thanks Jan, and thanks Olaf for the pointers to specific patches. I'll try
> to cherry pick those into my dom0 (debian mainline 3.0). Somebody else
> should get those in mainline though. Soonish :)

Well, could you post them once you have cherry-picked them?

Thanks.
> 
> Andres
> >>>> On 10.11.11 at 05:29, "Andres Lagar-Cavilla" <andres@lagarcavilla.org>
> >>>> wrote:
> >> The tools retry as long as IOCTL_PRIVCMD_MMAPBATCH_V2 is supported.
> >> Which
> >> it isn't on mainline linux 3.0, 3.1, etc.
> >
> > Seems like nobody cared to port over the code from the old 2.6.18 tree
> > (or the forward ports thereof).
> >
> >> Which dom0 kernel are you using?
> >
> > Certainly one of our forward port kernels.
> >
> >> And for backend drivers implemented in the kernel (netback, etc), there
> >> is
> >> no retrying.
> >
> > As above, seems like nobody cared to forward port those bits either.
> >
> > Jan
> >
> >
> 
> 
> 
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xensource.com
> http://lists.xensource.com/xen-devel

Re: Need help with fixing the Xen waitqueue feature

[George Dunlap]

On Wed, Nov 9, 2011 at 9:21 PM, Andres Lagar-Cavilla
<andres@lagarcavilla.org> wrote:
> PoD also does emergency sweeps under memory pressure to identify zeroes,
> that can be easily implemented by a user-space utility.

PoD is certainly a special-case, hypervisor-handled version of paging.
 The main question is whether a user-space version can be made to
perform well enough.  My guess is that it can, but it's far from
certain.  If it can, I'm all in favor of making the paging handle PoD.

> The hypervisor code keeps a list of 2M superpages -- that feature would be
> lost.

This is actually pretty important; Windows scrubs memory on boot, so
it's guaranteed that the majority of the memory will be touched and
re-populated.

> But I doubt this would fly anyways: PoD works for non-ept modes, which I
> guess don't want to lose that functionality.

Is there a particular reason we can't do paging on shadow code? I
thought it was just that doing HAP was simpler to get started with.
That would be another blocker to getting rid of PoD, really.

 -George